my friend is currently 6'1" ish and around 140. he doesnt have any kind of eating disorder or anything like that, just a very fast metabolism. now he wants to put on some weight. he works out regularly and is all muscle. is there anything he can do to increase his size? like take any supplements (like protein or something)?

I had the same problem till I whent over seas with my unit and one of my friends who is a personal trainer helped me out. First of all he taught me how to lift properly and get a good weight training program going. Lifted 5 out of 10 days. One hour at a time focusing on one body area each time, legs, arms, chest, back, shoulders were the breakdowns. Muscle and fitness have some awesome training manuals and logs for sale. Next everytime I ate I ate like I would never get a chance to eat again. Good food though, vegetables, lots of meats, ect. even dessert and sweets. Then I also drank protein shakes mixed with creatine. One in the morning, one after workout (muscles need food within one half hour or they'll eat themselves, and one before bed. Result I whent from 6"1" 145 lbs. to 188 lbs. of pure muscle in like less than 6 months! I did end up with some stretch marks though LOL. Oh my resting metabolic rate is like 2500. Very fast!